The ever growing importance of III-V compound semiconductor laser diodes in applications such as optical communications, printers and optical disk memory systems has created a high demand for high reliability devices that combine long lifetime with high power capabilities. Usually, maximum optical output power is limited by the catastrophic optical damage (COD) or output power saturation caused by local heating at the laser mirror facets. Thus, it is important that the COD level be raised by optimizing the mirror passivation to reduce heat development near the mirror interface.
Previously, most laser diodes with cleaved mirrors have displayed an excellent performance, high reliability and long lifetime at high optical power, provided that a proper passivation coating is applied to the mirror facets.
More recently, however, much attention has been directed to etched mirror devices mainly because the inherent advantages of this technology allows full wafer processing and testing as well as a high level of integration. A typical device and fabrication method are disclosed in the published European patent application 0 363 647 "Method for Etching Mirror Facets of III-V Semiconductor Structures." However, for etched mirrors it is more difficult to achieve the required reliability and long lifetime at the required output power. Thus, a definite need for improved devices clearly exists.
It has long been recognized that III-V compound surfaces, GaAs being the most widely studied, are generally of poor electronic quality, whereby the use of these materials in optoelectronic applications is limited. Considerable effort has been expended aiming at an improved understanding of the mechanisms that cause mirror heading and performance degradation. Today, it is generally believed that these problems are mainly caused by native oxides at the mirror interface and account for a large number of non-radiative recombination centers which, in turn, cause heating at the mirror facets.
Recently, a passivation method involving a sodium sulfide nonahydride (Na.sub.2 S.9H.sub.2 O) treatment has been found to favorably impact GaAs surface properties, one aspect being the reduction of minority carrier recombination centers. The key step of this method is the removal of native oxides from the semiconductor surface. Once Ga.sub.2 O.sub.3 and As.sub.2 O.sub.3 have been dissolved by a highly alkaline sulfide solution, the sulfur neutralizes the surface electronic states that otherwise form efficient recombination centers of minority carriers.
The effects and results of such sulfide treatments have been investigated and discussed in a number of publications:
"Structure and Stability of Passivating Arsenic Sulfide Phases at GaAs Surfaces" by C. J. Sandroff et al. (J. Vac. Sci. Technol. B7 (4), July/August 1989, pp. 841-843). Described are the results of an investigation on the use of aqueous sulfides on GaAs surfaces. The structure and degradation mechanism of the passivating phases formed during treatment with the aqueous sulfide was examined. Degradation was found to occur in the presence of oxygen and light, thereby producing a surface primarily composed of As.sub.2 O.sub.3. It is shown that long lasting passivation could be achieved if either light or oxygen is excluded. PA1 "Study of Novel Chemical Surface Passivation Techniques on GaAs pn Junction Solar Cells" by M. G. Mauk et al. (Appl. Phys. Lett. 54 (9), 16 January 1989, pp. 213-215) describes GaAs surface passivation techniques wherein passivation is achieved by chemical treatments using Na.sub.2 S, KOH, RuCI.sub.3 and K.sub.2 Se aqueous solutions. Solar cell structures are used to evaluate the effectiveness of these passivation techniques. PA1 "Effects of Passivating Ionic Films on the Photoluminescence Properties of GaAs" by B. J. Skromme et al. (Appl. Phys. Lett 51 (24), 14 Dec. 1987, pp. 2022-2024) describes the effects of passivating spin-coated Na.sub.2 S.9H.sub.2 O on GaAs surfaces. Photoluminescence methods are employed to characterize non-radiative surface recombination. After passivation, the photoluminescence was found to increase, and many of the surface field and surface recombination-related notch features eliminated. PA1 "Raman Scattering Measurements Of Decreased Barrier Heights in GaAs following Surface Chemical Passivation" by L. A. Farrow et al. (Appl. Phys. Lett. 51 (23), 7 Dec. 1987, pp. 1931 ff) discusses Raman scattering to provide a quantitative, contactless way of measuring the reduced barrier height associated with decreased density of GaAs surface states obtained by treating GaAs surfaces with thin films of sodium sulfide nonahydride (Na.sub.2 S.9H.sub.2 O)
The applicant is not aware of any publication disclosing or suggesting the application of such pre-passivation treatment in the fabrication of etched mirrors to improve laser reliability in view of the need for improved mirror facets, which has long been recognized.
As will be hereinafter described, the use of a pre-passivation treatment proceeded by a wet-etch "cleaning" process and followed by a deposition of a final passivation coating effectively prevents any chemical reaction at the mirror facet, providing the etched mirror laser devices with an unexpected high lifetime at optical powers not previously achieved.